1. Field of the Invention
The present invention relates to an optical fiber used, for example, in a long distance optical transmission system, more particularly relates to an optical fiber preform and an optical fiber formed by the preform which improves the profile of the refractive index of the core portion to an ideal shape and to a process of producing the same.
2. Description of the Related Art
For long distance optical transmission systems, single mode optical fibers with a core portion with an extremely reduced amount of doping of GeO.sub.2 by which the Rayleigh scattering coefficient is reduced, has been used. Usually fluorine is doped into a cladding portion of the fiber for the purpose of increasing the difference in refractive index with the core portion.
In such a single mode optical fiber, the profile of the refractive index of the core portion desirably should have a step-like profile compared with the cladding portion. This is in particular to prevent an increase in the optical transmission loss with respect to bending of the optical fiber.
However, in the conventional optical fibers and processes of production thereof, it was not possible to make the profile of the refractive index of the core portion the ideal step shape. As shown in FIG. 1, a core portion 2 ended up with a profile of the refractive index which sticks up in a gentle mountain form compared with a cladding portion 4. Then, the optical fiber suffers from the problem that the effective refractive index of the core portion dropped and the optical transmission loss with respect to bending of the fiber increased.
Note that the reason why the profile of the refractive index as shown in FIG. 1 is obtained is considered to be because the GeO.sub.2 included in the core portion vaporizes from the surface of the soot during dehydration step and diffuses to the cladding portion at the interface of the core portion and cladding portion during the transparent glassification step in the process of production of the optical fiber. Therefore, it is extremely difficult to dope GeO.sub.2 uniformly in the core portion and obtain a step-like refractive index profile. Note that the dotted line portion A in FIG. 1 is a straight line corresponding to the refractive index in the case of SiO.sub.2 alone without any doping.